Adaptive filters operate on an incoming signal in accordance with a prescribed algorithm to generate a desired output signal. Typically, the filters generate an impulse response in accordance with the algorithm which includes adjustment of so-called tap coefficients to update the impulse response in response to an error signal. In this way the filter impulse response is optimized to yield the desired result.
During certain incoming signal conditions, for example, partial band energy including single frequency tones, multifrequency tones and the like, the tap coefficients tend to drift toward relatively large values, which is undesirable. This condition has been countered by introducing a so-called leakage signal into the tap coefficient signals which tends to drive the tap coefficient signals toward zero. Prior tap coefficient leakage arrangements are disclosed in U.S. Pat. Nos. 4,237,554 issued Dec. 2, 1980 and 4,243,959 issued Jan. 6, 1981.
Both of the prior arrangements, however, introduce leakage at substantially a constant rate during all incoming signal conditions. Specifically, the arrangement disclosed in U.S. Pat. No. 4,243,959 introduces a so-called "weak" leakage by periodically or intermittently switching the supply of a leakage signal to the tap coefficient signals.
I have determined that in the prior arrangements the value of the leakage signal supplied to the tap coefficients is a compromise in order to minimize deleterious effects during intervals including other than partial band energy in the incoming signal. It has been determined that a relatively large leakage is needed during intervals when partial band energy is incoming to the filter and relatively small or no leakage is needed during intervals when whole band energy, for example, speech, noise and the like, is incoming to the filter.